Fullerene surfactants and their use in polymer solar cells

ABSTRACT

Fullerene surfactant compounds useful as interfacial layer in polymer solar cells to enhance solar cell efficiency. Polymer solar cell including a fullerene surfactant-containing interfacial layer intermediate cathode and active layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 13/706,230, filed Dec. 5, 2012, which claims the benefit of U.S. Application No. 61/566,943, filed Dec. 5, 2011, each application is expressly incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under Contract Nos. FA2386-11-1-4072 awarded by the Air Force Office of Scientific Research, N00014-11-1-0300 awarded by the Office of Naval Research, and DE-FC3608GO18024/A000 awarded by the Department of Energy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Effective control of organic-metal interfaces is critical for achieving high-performance polymer solar cells (PSCs). Ideally, the work-function (F) of the cathode and anode should be aligned with the energy of the photo-excited quasi-Fermi levels (E_(F)) of organic semiconductors to create Ohmic contact for maxing achievable open-circuit voltage (V_(oc)) and minimized energy barrier for charge-extraction. Although low Φ metal such as Ca (Φ=2.9 eV) has been proved to form good contact with bulk heterojunction (BHJ) layer as cathode, its vulnerability to environmental conditions undermines its use for practical applications. More stable metals like Al (Φ=4.28 eV) and Ag (Φ=4.57 eV) have been used as cathode, but their relatively high Φ often cause energy mismatch between BHJ blends and themselves, which results in lower V_(oc) and device performance.

To alleviate this problem, proper interfacial engineering by inserting a thin layer between cathode and active layer has been vigorously explored. For example, inorganic materials such as LiF and Cs₂CO₃ and metal oxides (TiO_(x), ZnO_(x)), and organic materials such as insulating poly(ethylene oxide) (PEO) and conjugated polyelectrolyte (CPE) have also been proved to be effective in improving Al cathode based device performance. In a recent study, 8.37% of PCE was reported by inserting polyfluorene derivative (PFN) between the high performance PTB7:PC71BM BHJ and Ca/Al. In addition, self-assembled fullerenes (e.g., PCBM capped PEG and fluorocarbon modified PCBM (F-PCBM)) have also been reported to increase P3HT:PCBM based device performance.

Despite that interface engineering has been performed for conventional PSCs, the performances obtained from Ag-based devices were usually lower than those using Ca/Al and Al cathode. This significantly limits the utilization of stable and reflective Ag as cathode for improving performance and stability of devices, though it is well-known Ag anode can be advantageous in inverted PSCs to facilitate the printing process.

On the other hand, fullerene-based materials not only can match well with the energy level of the lowest unoccupied molecular orbital (LUMO) of commonly used acceptor (e.g., PCBM), but also possess sufficiently deep highest occupied molecular orbital (HOMO) energy level, which make them as energetically ideal candidates for electron transport layer (ETL) to facilitate electron-selecting and hole-blocking in PSCs.

Despite the advances in the development of materials to enhance solar cell performance, a need exists to provide effective interfacial materials that are capable of adjusting the Φ of cathode to improve the contact with the BHJ layer, possess reasonable electron mobility to minimize electrical resistance across the interfacial layer, and have sufficient orthogonal solvent-processability and film forming properties to avoid eroding into the BHJ layer. The present invention seeks to fulfill this need and provides further related advantages.

SUMMARY OF THE INVENTION

The present invention provides fullerene surfactant compounds that can be incorporated into polymer solar cells as an interfacial layer intermediate the cells' active layer and cathode to enhance solar cell efficiency.

In one aspect the invention provides a fullerene compound, comprising:

(a) a fullerene group;

(b) one or more cationic nitrogen centers covalently coupled to the fullerene group;

(c) one or more hydrophilic groups covalently coupled to the fullerene group; and

(d) one or more counter ions associated with the cationic nitrogen center.

Representative fullerene groups include C₆₀, C₇₀, C₇₆, C₇₈, C₈₂, C₈₄, and C₉₂ fullerene groups. In one embodiment, the fullerene group is a C₆₀ fullerene group. In one embodiment, the cationic nitrogen center is a quaternary amine group. Suitable hydrophilic groups include polyether and polyol groups. In certain embodiments, the polyether group is a polyalkene oxide group such as a polyethylene oxide group having the formula —(CH₂CH₂O)_(n)—, where n is from 1 to about 20. In certain embodiments, the fullerene compound further includes comprising an anionic center. Representative anionic centers include sulfonate (SO₃ ²⁻) and carboxylate (—CO₂ ⁻) groups. In one embodiments, the fullerene compound is a mono-fulleropyrrolidium. In other embodiment, the fullerene compound is a bis-fulleropyrrolidium.

In one embodiment, the compound has the structure:

In another embodiment, the compound has the structure:

In these embodiments, F is a fullerene group; B is a N-containing ring having from 5-7 ring atoms; R₁ and R₂ are independently selected from the group consisting of a polyalkylene oxide and a C1-C20 alkyl optionally substituted with an anionic center; Ar is —C₆H₅-PEO, wherein —C₆H₅-PEO is selected from the group consisting of mono-, di-, tri-, and tetra-PEO substituted phenyl; and A⁻ is a counter ion associated with the cationic nitrogen center.

In another aspect of the invention, photovoltaic devices are provided. In certain embodiments, the photovoltaic device includes an interfacial layer intermediate the cathode and active layer, wherein the interfacial layer includes one or more fullerene surfactant compounds of the invention. In one embodiment, the photovoltaic device includes:

(a) a first electrode;

(b) an active layer disposed on a surface of the first electrode;

(c) a layer comprising a fullerene compound of the invention disposed on a surface of the active layer opposite the first electrode; and

(d) a second electrode disposed on a surface of the layer comprising the fullerene compound of the invention opposite the active layer.

In another embodiment, the device further includes a hole transport layer intermediate the first electrode and the active layer.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings.

FIG. 1 illustrates the structures of representative fullerene surfactants of the invention.

FIG. 2 is a schematic illustration of the preparation of two representative fullerene surfactants of the invention, ETL-1 and ETL-2.

FIG. 3 is a cross-sectional view of a representative photovoltaic device of the invention incorporating a fullerene surfactant-containing interfacial layer intermediate the cathode and the active layer.

FIG. 4A is a schematic illustration of the use of two representative fullerene surfactants of the invention, ETL-1 and ETL-2, as interfacial layers in a PIDT-PhanQ:PC₇₁BM polymer solar cell device. FIG. 4B is a schematic energy diagram of the device shown in FIG. 4A: PIDT-PhanQ:poly(indacenodithiophene-co-phananthrene-quinoxaline) PC₇₁BM: [6,6]-phenyl C71-butyric acid methyl ester.

FIGS. 5A-5F compare the current density-voltage (J-V) characteristics of devices under illumination of AM 1.5 G at 100 mW cm⁻² for Al, Ca/Al, and Ag cathodes (FIGS. 5A, 5C, and 5E), respectively, and their corresponding external quantum efficiency (EQE) spectra (FIGS. 5B, 5D, and 5F).

FIG. 6 compares surface morphology (5 μm×5 μm) and surface profile (10 nm to −10 nm) of PIDT-PhanQ:PC₇₁BM BHJ based device: (a) BHJ only, (b) ETL-1 on BHJ, (c) ETL-2 on BHJ. RMS roughness for (a) 0.733 nm, (b) 1.01 nm, (c) 0.763 nm, respectively.

FIG. 7 shows the chemical structure of a representative fullerene surfactant of the invention: ETL-2 (“C₆₀-bis”).

FIG. 8A is a schematic illustration of the architecture of a representative photovoltaic device of the invention. FIG. 8B is a schematic illustration of the energy level diagram of the device of FIG. 8A.

FIGS. 9A-9D compare performance data for PIDT-PhanQ:PC₇₁BM devices fabricated with different choice of cathode metal with and without a C₆₀-bis interlayer. The current density-voltage curves (9A) and external quantum efficiency spectra (9B) show increases in V_(OC) and J_(SC) respectively. Capacitance-voltage (9C) and Mott-Schottky (9D) analysis explain increased V_(OC) in terms of the V_(BI) of the Schottky contact.

FIG. 10 compares normalized PCE for Al, Ag, and Cu devices with and without C₆₀-bis under ambient conditions.

FIG. 11 is a secondary electron cutoff spectrum and first derivative of Ar⁺ ion sputter-cleaned Au foil represented on the kinetic energy scale. The vertical line through the center of the first derivative is a guide for reading the work function directly from the kinetic energy scale.

FIGS. 12A-12C compare secondary electron cutoff spectra of Al (12A), Ag (12B), and Cu (12C) metal films with and without C₆₀-bis. Films without C₆₀-bis were Ar⁺ sputter-cleaned in vacuo prior to measurement. The Cu spectrum includes that of clean Au foil as a reference.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides fullerene surfactants and their use to modify the interface of the cathode and bulk heterojunction layer in organic solar cells. The incorporation of an interfacial layer including a fullerene surfactant of the invention in a conventional polymer solar cell enhances the efficiency of the solar cell.

In one aspect, the invention provides a fullerene surfactant. As used herein, the term “fullerene surfactant” refers to a fullerene that includes hydrophilic group sufficient to render the fullerene solution processable in the fabrication of polymer solar cells. The fullerene surfactant includes a fullerene group, one or more cationic amine centers, one or more hydrophilic groups, and one or more counter ions. The cationic amine group is covalently coupled to the fullerene group. The hydrophilic group is covalently the fullerene group. In certain embodiments, the hydrophilic group is covalently coupled to the fullerene group through the cationic amine group. In certain embodiments, the fullerene surfactant further includes an anionic center.

Representative fullerene groups include C₆₀, C₇₀, C₇₆, C₇₈, C₈₂, C₈₄, and C₉₂ fullerene groups. In one embodiment, the fullerene surfactant of the invention includes a C₆₀ group.

The cationic amine group is a positively-charged amine center. Suitable cationic amine groups include quaternary amine groups prepared by quaternizing amine precursor compounds. In certain embodiments, the fullerene surfactants of the invention are prepared by quaternization of precursor fullerene amine compounds.

Representative hydrophilic groups include one or more hydrophilic substituents such as ether and alcohol groups. In certain embodiments, the hydrophilic group is a polyether group. Representative polyether groups include polyalkylene oxides with as polyethylene oxide (PEO) groups, polypropylene oxide (PPO) groups, and groups that include ethylene oxide and propylene oxide groups. Suitable polyethylene oxide groups have the formula —(CH₂CH₂O)_(n)—, where n is from 1 to about 20, and —(CH(CH₃)CH₂O)_(n)—, where n is from 1 to about 20. In other embodiments, the hydrophilic group is a polyol.

In embodiments of the fullerene surfactants that include anionic centers, representative anionic centers include sulfonate (SO₃ ²⁻) and carboxylate (—CO₂ ⁻) groups. The anionic centers are covalently coupled to the fullerene group.

Representative fullerene surfactants of the invention are illustrated in FIG. 1. Referring to fullerene surfactant compounds F1-F14 in FIG. 1, the fullerene group may be any one of C₆₀, C₇₀, C₇₆, C₇₈, C₈₂, C₈₄, and C₉₂ fullerene groups; R is independently selected from the group consisting of C1-C20 straight chain and branched alkyl; PEO is an alkylene oxide group independently selected from the group consisting of polyethylene oxide having the formula —(CH₂CH₂O)_(n)—, where n is from 1 to about 20 or polypropylene oxide having the formula —(CH(CH₃)CH₂O)_(n)—, where n is from 1 to about 20; —C₆H₅-PEO is selected from the group consisting of mono-, di-, tri-, and tetra-PEO substituted phenyl; and A⁻ is a counter ion selected from the group consisting of fluoride, chloride, bromide, iodide, trifluoromethyl sulfonyl (CF₃SO₃ ⁻), tetrakis(imidazolyl)borate (BIm₄ ⁻), and tetrakis(3,5-bis(trifluoromethyl)phenyl]borate (TFPB⁻).

In one embodiment, the fullerene surfactant compounds of the invention have formula (IA):

In another embodiment, the fullerene surfactant compounds of the invention have formula (IB):

In a further embodiment, the fullerene surfactant compounds of the invention have formula (IIA):

In another embodiment, the fullerene surfactant compounds of the invention have formula (IIB):

In one embodiment, the fullerene surfactant compounds of the invention have formula (III):

In another embodiment, the fullerene surfactant compounds of the invention have formula (IV):

In one embodiment, the fullerene surfactant compounds of the invention have formula (V):

In another embodiment, the fullerene surfactant compounds of the invention have formula (VI):

For fullerene surfactant compounds noted above (i.e., compounds of formula (IA)-(VI)), F is a fullerene group (e.g., C₆₀, C₇₀, C₇₆, C₇₈, C₈₂, C₈₄, and C₉₂); B is a N-containing ring fused to the fullerene group and having from 5-7 ring atoms (e.g., pyrrolidine, a 5-membered ring); R₁ and R₂ are independently selected from the group consisting of a polyalkylene oxide (e.g., PEO or PPO), as described above, and a C1-C20 alkyl optionally substituted with an anionic center (e.g., sulfonyl or carboxyl); Ar, Ar₁, and Ar₂ are independently selected from the group consisting of —C₆H₅-PEO, —C₆H₅—N⁺(PEO)₂R₁|A⁻; and —C₅H₄N⁺—R₁|A⁻, wherein —C₆H₅-PEO is selected from the group consisting of mono-, di-, tri-, and tetra-PEO substituted phenyl, wherein —C₆H₅—N⁺(PEO)₂R₁ is a substituted aniline, and wherein —C₅H₄N⁺—R₁ is a substituted pyridinium; L₁ and L₂ are linkers having from 1 to 20 carbon atoms (e.g., C1-C20 alkylene) optionally including one or more heteroatoms (e.g., O, N, or S) and/or one or more functionalized carbon atoms (e.g., C═O); and A⁻ is a counter ion associated with the cationic nitrogen center.

The preparation of two representative fullerene surfactants of the invention, ETL-1 and ETL-2, is illustrated schematically in FIG. 2 and described in Example 1.

By virtue of its component groups, the fullerene surfactant of the invention is advantageously soluble in a solvent orthogonal to the device active layer. In practice of the method of the invention, device fabrication includes forming a layer intermediate the active layer and cathode. Application of the fullerene surfactant to the active layer provides a fullerene surfactant layer onto which the cathode is formed.

FIG. 3 is a cross-sectional view of a typical heterojunction photovoltaic device in accordance with one embodiment of the invention. Referring to FIG. 3, photovoltaic device 100 includes first electrode 110 (anode), hole transport layer 120 (also referred to as a charge transport or charge selective layer) formed on first electrode 110, photovoltaic layer 130 (also referred to as the active layer) formed on charge transport layer 120, fullerene surfactant-containing layer 140 (also referred to as electron transport or electron selective layer and also referred to herein as the “interfacial layer”) formed on photovoltaic layer 130, and second electrode 150 (cathode) formed on fullerene surfactant-containing layer 140. Photovoltaic layer 130 is the active layer, such as a BHJ layer.

In the devices of the invention, the hole transport and electron transport layers define the charge collection properties in the devices. The best devices reported to date are composed of a layer of polymer donor and fullerene acceptor bulk-heterojunction (BHJ) film sandwiched between a transparent electrode, such as indium tin oxide (ITO), and a metal electrode. Under illumination, photo-generated excitons will dissociate at the donor-acceptor interface, driven by the difference in energy levels between the two semiconductors. The separated charges will then drift under the inherent electric field created by the work-function difference between the asymmetric electrodes and ultimately, will be collected by the corresponding electrodes. The PCE is defined by the product of three parameters including short-circuit current density (J_(sc)), open-circuit voltage (V_(oc)), and fill factor (FF).

The nature of electrical contact between the active BHJ layer and the electrodes can significantly affect all three device-related parameters and modification of those interfaces by inserting appropriate interfacial layers can significantly alter the contact properties to improve the PCE of OPVs. The interfacial layer of the invention serves multiple functions that include:

(a) tuning the energy level alignment at the electrode/active layer interface;

(b) defining polarity of electrodes and improving charge selectivity;

(c) controlling surface properties to alter the morphology of the active layer;

(d) introducing optical spacer and plasmonic effects to modulate light absorption in the active layer; and

(e) improving interfacial stability between the active layer and electrodes.

The photovoltaic layer (or active layer) can include any one of a variety of materials and mixtures of materials as known in the art. Representative useful materials include P3HT, PIDT-PhanQ, PECz-DTQx, PCDTBT, PDTSTPD, PDTGTPD, PTB7. Representative active fullerene materials include PCBM and ICBA. Other representative active fullerene materials suitable for inclusion in a photoactive layer include those described in U.S. Patent Application Publication No. US 2011/0132439, incorporated herein by reference in its entirety.

The following is a description of representative fullerene surfactants of the invention and their use in interfacial layers to enhance the efficiency of polymer solar cells.

The present invention provides fullerene surfactants, ETL-1 and ETL-2, that can be readily dissolved in alcoholic solvents and applied as interfacial layer for cathode (see FIG. 4A), which exhibited effective tuning of cathode Φ, extraction of electrons, and photocurrent generation in devices. These two fullerene surfactants intrinsically help forming interfacial contact between metal (in either high or low F) and BHJ to improve device performance. Recently, the mechanism of using fullerene surfactant to enhance device V_(oc) has been elucidated that the metal E_(F) is pinned to the LUMO energy level of interfacial layer, thus increasing the device's V_(oc) regardless of the choice of different cathode metal. The present invention provides an organic interfacial material to realize Ag cathode based OPV with superior performance (as high as 6.63%) to those of Ca/Al and Al based devices due to the solvent-processed fullerene ETL simultaneously enhanced V_(oc), J_(sc), and FF of device.

ETL-1 and ETL-2 having compact integration of both ionic moieties and polar ethylene oxide chains onto a C₆₀ core were prepared by quaternizing the tertiary nitrogen of fulleropyrrolidines with methyl iodide (FIG. 2). Comparing to the poor solubility of most fullerene derivatives (e.g., Mono 1, Bis 2 and PCBM) in polar solvents, fulleropyrrolidiniums (ETL-1 and ETL-2) exhibit amphiphilic properties that can be dissolved in both chloroform and methanol, which provides great flexibility for processing in orthogonal-solvents to prevent eroding bottom BHJ layer.

The energy levels of ETL-1 and ETL-2 were estimated by cyclic voltammogram measurements. As shown in Table 1, the LUMOs of ETLs _exhibit small energy-gradient compared to that of PCBM due to that the electron-deficient cationic nitrogen is in close vicinity of the fullerene core, which made this interfacial material energetically favor electron collection and transport from PCBM to cathode.

TABLE 1 Reduction Potentials and estimated LUMO for fullerene surfactants.^(a) E_(1/2) ^(red) vs. Fc/Fc⁺ LUMO^(b) LUMO^(c) E₁(V) E₂(V) E₃(V) (eV) (eV) ETL-1 0.87 1.39 2.05 3.93 4.44 ETL-2 0.90 1.43 — 3.90 4.39 PC₇₁BM 0.99 1.53 2.04 3.81 4.30 ^(a)Potential in volt vs. a ferrocene/ferrocenium couple. ^(b)The LUMO levels were estimated using the following equation: LUMO level = −(4.8 + E_(1/2) ^(red1)) eV. ^(c)correlated LUMOs according to PCBM standard (LUMO = −4.30 eV).

Both ETL surfactants possess reasonable electron motilities (2.18×10⁻⁴ cm² V⁻¹ s⁻¹ for ETL-1 and 4.91×10⁻⁶ cm² V⁻¹ s⁻¹ for ETL-2), and show negligible absorbance to visible light, which qualify them as proper electron-transporting layer (ca. 10 nm). ETL-1 and ETL-2 bearing cationic nitrogen and PEO linkage effectively up-shifted the Φ of Al and Ag, around 0.8 eV by X-ray photoelectron spectroscopy (XPS) studies. It may be due to the polar interaction between fullerene surfactants and metal facilitate pinning of the metal E_(F) to that of the ETLs upon equilibration, which reduced energy barrier between BHJ layer and cathodes. This, in turn, increases V_(oc) and charge extraction efficiency.

The presence of these fullerene layers creates only minimal energy barrier height for electron extraction from PCBM (due to matched ETL LUMOs to that of PCBM). This is different from using the insulating PEO and p-type CPE process that have unfavored energy level and charge-transporting properties. Moreover, the n-type nature of fullerene surfactant layer creates an extra acceptor-donor junction that can potentially enhance exciton dissociation and prevent cathode from forming direct contact with active layer to quench excitons. These rationale are supported, vide infra, by the enhanced performance of PSCs with spun interfacial layers.

PSCs with fullerene surfactant-modified Al were studied. Device configuration of ITO/PEDOT:PSS/PIDT-PhanQ:PC₇₁BM/ETL/Al (FIG. 4A) showed significantly improved V_(oc), J_(sc) and FF compared to those from the reference device without the interfacial layer (FIGS. 5A, 5B, and Table 2). The reference device A with bare Al as cathode showed a relatively low PCE of 3.54% with a V_(oc) of 0.61 V, a J_(sc) of 10.55 mA cm⁻², and a FF of 0.55. The Schottky-barrier at the active layer/Al interface caused low V_(oc), J_(sc), and FF. However, when a thin layer (about 10 nm) of ETL-1 or ETL-2 was inserted between the BHJ layer and Al, higher PCE of 5.96% (device B) and 6.03% (device C) could be achieved, which accounts for a 70% improvement for device C compared to the reference device. All the parameters (J_(sc), V_(oc) and FF) increased significantly for devices B and C, because a better interfacial contact was created between BHJ and Al when the fullerene ETL was applied, which lowered the Φ of Al, thus giving higher V_(oc), and efficient electron extraction to give higher J_(sc) and FF.

TABLE 2 Characteristics of Devices A-F. V_(oc) J_(sc) PCE Device Cathode [V] [mA/cm²] FF [%] A Al 0.61 10.55 (10.34) 0.55 3.54 B ETL-1/Al 0.86 11.17 (11.09) 0.62 5.96 C ETL-2/Al 0.86 11.31 (11.27) 0.62 6.03 D Ca/Al 0.86 11.08 (10.64) 0.63 6.00 E ETL-1/Ca/Al 0.87 11.12 (11.07) 0.64 6.19 F ETL-2/Ca/Al 0.88 11.36 (11.20) 0.65 6.50 G Ag 0.74 10.92 (10.96) 0.57 4.61 H ETL-1/Ag 0.87 11.28 (11.11) 0.64 6.28 I ETL-2/Ag 0.88 11.41 (11.30) 0.66 6.63

The values in parentheses were calculated from EQE spectrum.

To further understand the effect on surfactant-modified cathodes, the commonly adopted Ca/Al cathode based device were also studied, in the device configuration of ITO/PEDOT:PSS/PIDT-PhanQ:PC₇₁BM/ETL/Ca/Al. Good contact between Ca/Al cathode and BHJ can give essentially high-performance, 6% PCE of device D (V_(oc)=0.86 V, J_(sc)=11.08 mA cm⁻², and FF=0.63). Slight improvements of device characteristics could be observed when the ETL layer was applied (FIG. 5C, 5D, and Table 2). Improved PCE of 6.19% (ETL-1, device E) and 6.50% (ETL-2, device F) were achieved, which correlated to the slightly enhanced V_(oc), J_(sc), and FF. These results indicated that the additional fullerene ETL layer helped optimize the contact between Ca/Al and BHJ layer leading to increased PCE. Although being widely used in PSCs as electrodes, Al and Ca/Al are sensitive to air and moisture, which cause device degradation in ambient. Ag shows relatively good stability toward ambient condition. However, the energy mismatch between high Φ of Ag and LUMO of PCBM usually resulted in poor device performance. To alleviate this problem, devices were fabricated with the configuration of ITO/PEDOT:PSS/PIDT-PhanQ:PC₇₁BM/ETL/Ag (devices G-I). A distinctly improved PCE (44%) could be achieved for device I compared to that of reference device G due to enhanced V_(oc), J_(sc), and FF (FIGS. 5E, 5F, and Table 2). PCE of 6.63% from device I using ETL-2 is one of the highest values achieved from conventional PSC with Ag cathode.

The external quantum efficiency (EQE) spectra of devices A-I (FIGS. 5B, 5D, and 5F) were compared. The calculated J_(sc) obtained from integration of EQE spectrum match well with measured one, with variation of less than 5% (Table 2). With an ETL-1 or ETL-2, the EQE of devices are higher in part of the spectrum compared to those of reference device, which correlate well with the results of higher photocurrents.

In all devices, ETL-1 show slightly lower PCE and relevant parameters (J_(sc) and FF) than those of ETL-2, which may be due to the difference of film quality of these two ETLs on top of the BHJ layer. The topography and surface profile of devices with and without ETL layer were characterized by atomic force microscopy (AFM) and is shown in FIG. 6. All the interfacial layers covered well on top of BHJ layer. The surface of ETL-2 is relatively smooth as indicated by the lower root-mean-square (RMS) roughness, 0.763 nm (FIG. 6( c)) and is similar to 0.733 nm of BHJ surface (FIG. 6( a)). ETL-1 on BHJ exhibited a RMS of 1.01 nm with relatively rough surface (FIG. 6( b)).

In one aspect, the invention provides representative fullerene surfactants, ETL-1 and ETL-2, which can be readily processed in orthogonal solvents (e.g., methanol) on a BHJ layer in PSCs. These materials possess proper electron mobility and the capability of tuning cathode Φ to improve electron extraction and photocurrent generation. Upon the insertion of a thin ETL-1 or ETL-2 between various metal cathodes and BHJ layer (device A-I), simultaneously improved V_(oc), J_(sc), and FF could be achieved for these devices compared to those without using surfactant. The performance of PSCs is significantly improved (70% for Al cathode and 40% for Ag cathode) when surfactant-modified cathode was applied. High performance PSCs using fullerene ETL modified Ag cathode were realized (as high as 6.63%) which is superior to those of Ca/Al and Al based devices.

The following is a description of the use of a representative fullerene surfactant of the invention, ETL-2 (“C₆₀-bis”) (FIG. 7), in an interfacial layer to enhance the efficiency of polymer solar cells. FIG. 8A is a schematic illustration of the architecture of a representative photovoltaic device of the invention. FIG. 8B is a schematic illustration of the energy level diagram of the device of FIG. 8A.

Devices were fabricated with higher WF metals less prone to oxidation, which are shown to perform better than Al devices over time. Remarkably, the V_(OC) appears to be independent of the choice of cathode metal when C₆₀-bis is used as a buffer layer.

FIG. 9A shows the J-V characteristics for devices fabricated with different cathode metals both with and without a C₆₀-bis buffer layer. The V_(OC) for devices with an Al cathode is consistently lower than that of Cu and Ag devices, which can be attributed to the rapid oxidation of Al in air. The non-ideal nature of this interface also manifests in a modest fill factor (FF) of 0.51 and an overall PCE of 3.22%. In contrast, when a layer of C₆₀-bis is used, the PCE increases to 5.87% as a result of an increase in J_(SC), FF and most notably V_(OC). In addition, the shunt resistance is shown to increase for all metals in the case of C₆₀-bis, which provides evidence of lower leakage current under illumination. Performance data for all devices are summarized in Table 3.

TABLE 3 Performance data for PIDT-PhanQ:PC₇₁BM devices with different cathode metals, with and without C₆₀-bis. V_(OC) J_(SC) PCE R_(SH) Device [V] [mA cm⁻²] FF [%] [Ω cm⁻²] Al 0.62 10.28 0.51 3.22 309.33 Al/C₆₀-bis 0.88 11.19 0.60 5.87 773.33 Ag 0.73 10.83 0.53 4.22 351.63 Ag/C₆₀-bis 0.88 11.50 0.61 6.22 662.86 Cu 0.67 9.58 0.51 3.32 386.67 Cu/C₆₀-bis 0.87 10.13 0.61 5.37 795.43

To investigate the improvement in J_(SC), external quantum efficiency (EQE) spectra (FIG. 9B) were obtained for Al, Ag, and Cu devices. The spectra exhibit an almost constant increase across the entire wavelength range for each case when the surfactant layer was inserted. This indicates the improvement in J_(SC) is due entirely to the inclusion of the surfactant and a concurrent decrease in recombination resistance at the organic/electrode interface, rather than a change in bulk morphology.

To further demonstrate the utility of C₆₀-bis as an interfacial layer, the PCE of devices with different cathode metals were tracked over a period of time under exposure to ambient conditions. FIG. 10 shows the normalized PCE for unencapsulated devices with and without C₆₀-bis over 100 h in air. As expected the performance of Al devices drops off rapidly, even with the inclusion of the fullerene surfactant, which is likely due to the uptake of oxygen and water molecules and their subsequent diffusion to the metal/organic interface. The Ag and Cu devices remain very stable, however, with the Cu/C₆₀-bis retaining nearly 90% of its original PCE after the entire period of ambient exposure.

By far the most obvious benefit of C₆₀-bis is a strongly enhanced V_(OC). To further investigate the dramatic increase in V_(OC) when C₆₀-bis is used, capacitance-voltage characteristics (C-V) were obtained and devices were analyzed via Mott-Schottky (MS) analysis. It has previously been shown that, due to the intrinsic p-doped nature of semiconducting polymers, a Schottky contact is formed upon deposition of the cathode onto the photoactive layer. The depletion zone formed at this interface is modulated by the applied voltage under reverse and low (<1.5V) forward bias. Band-bending has been shown to result in the vicinity of the cathode, allowing extraction of the built-in potential (V_(BI)) and impurity concentration (N) of the region by application of C⁻²=(2/qN)(V_(BI)−V) to the appropriate bias voltage range.

FIG. 9C shows the capacitance behavior of all devices as a function of bias voltage. The low capacitance region up to about 0.5 V has been attributed to the capacitance of the depletion layer, whereas a further increase in forward bias voltage yields a peak in the capacitance related to the storage of minority carriers in the bulk. FIG. 9D shows the MS plot for all fabricated devices. At moderate to high reverse bias, C⁻² tends to reach a steady value related to the geometric capacitance of the organic material which has become fully depleted of majority carriers and can be viewed as a classical dielectric. The linear region under low forward bias is related to the formation of a Schottky contact and can be fitted to a plot of C⁻² versus bias voltage. Extrapolation of the linear fit line to the intercept on the bias axis directly yields V_(BI) for the device. Once a value for V_(BI) has been obtained, an impurity concentration N and depletion width w=(2∈V_(BI)/qN)^(1/2) corresponding to zero applied bias can be extracted. A dielectric permittivity of 3 has been assumed for calculations involving these equations. MS analysis data, along with the relative shifts in V_(OC) and V_(BI), are summarized in Table 4.

TABLE 4 Built-in potential V_(BI), dopant concentration N, and depletion width w of the organic/cathode Schottky contact from Mott-Schottky analysis. The work functions and relative shifts in V_(OC) and V_(BI) for all devices are also included. (ΔV_(OC), V_(OC) V_(BI) ΔV_(BI)) N w Φ_(cathode) Device [V] [V] [V] [10¹⁶ cm⁻³] [nm] [eV] Al 0.619 0.636 — 2.25 97 4.25 Al/C₆₀-bis 0.877 0.940 (0.26, 0.30) 3.32 97 3.66 Ag 0.734 0.808 — 3.39 89 4.57 Ag/C₆₀-bis 0.879 0.959 (0.15, 0.15) 3.77 92 3.97 Cu 0.672 0.712 — 2.51 97 4.70 Cu/C₆₀-bis 0.875 0.957 (0.20, 0.25) 4.02 89 3.96

The depletion width extracted from the capacitance-voltage data extends over almost the entire thickness of the active layer. When taken with the N values obtained from the same data, this indicates a consistent doping profile across the entire layer that changes negligibly by inclusion of C₆₀-bis. Because the change in the Fermi level of the active layer (E_(F) ^(p)) can be approximated by ΔE_(F) ^(p)=k_(b)T ln(N_(b)/N_(a)), where N_(b) and N_(a) are the dopant concentrations of the device with and without C₆₀-bis, respectively, it is reasonable to conclude that E_(F) ^(p) does not change more than ca. 10 meV. When a semiconductor is placed in intimate contact with a metal, their respective E_(F) come into equilibrium by electrons being transferred “downhill” in energy. Referencing V_(BI) to E_(F) ^(p) by V_(BI)=(E_(F) ^(p)−Φ_(cathode)), where Φ_(cathode) is the cathode WF, then the difference in V_(BI) with and without C₆₀-bis can be attributed to a modification of Φ_(cathode) by the surfactant. Furthermore, because the relative shifts in V_(BI) closely follow those of V_(OC) for all three metals we can conclude that the observed increase in V_(OC) upon inclusion of C₆₀-bis is due to a dipole-induced shift in Φ_(cathode) at the interface.

To further investigate the energetics at the interface, WFs were obtained for Al, Ag, and Cu with and without C₆₀-bis spin-coated on top and are summarized in Table 4. WFs of in-situ, sputter-cleaned Al, Ag, and Cu films were measured to be 4.25 eV, 4.57 eV and 4.70 eV, respectively (FIGS. 12A-12C). The WFs of Ag and Cu with C₆₀-bis yield nearly the same value. Because sampling of the substrate at normal emission is highly surface sensitive, it is reasonable to assume these WF values correspond to the C₆₀-bis. As the material is an n-type semiconductor, one would expect E_(F) to be closer to the LUMO level than mid-gap. It should be noted that the WFs of the organic overlayer may not be measured in the flat-band condition, but are rather subject to any band bending occurring at the metal/organic interface as a result of E_(F) equilibration. Additionally, it is likely that an unavoidable thin oxide layer formed on the Al sample when it was removed from the glovebox for C₆₀-bis deposition, as evidenced by a comparison of O1s peak intensity in XPS survey spectra for bare Al before and after sputter-cleaning with Ar⁺ ions. These considerations might explain the lower WF of the modified Al cathode as compared to Ag and Cu.

It should be stressed that these conditions do not prevail for regular device fabrication since the cathode is deposited under high vacuum after spin-coating the C₆₀-bis layer outside the glovebox. Regardless, at a distance sufficiently far into the bulk of the photoactive layer only the effective WF of the C₆₀-bis modified cathode can be seen by the rest of the device. This ensures a constant difference between E_(F) ^(p) and Φ_(cathode), and explains why V_(BI), and consequently V_(OC), is nearly the same for all three metals when C₆₀-bis is employed.

A C₆₀ bis-adduct surfactant was used to modify the energy level alignment at the organic/cathode interface in conventional structure, bulk-heterojunction OSC devices. A well-defined interface between the photoactive layer and the surfactant was ensured by virtue of process solvent orthogonality. The large increase in device V_(OC) is independent of the choice of cathode metal due to pinning of the metal E_(F) to that of the C₆₀-bis upon equilibration. Mott-Schottky analysis of the interface formed between the photoactive layer and the cathode yields a built-in potential defined by the difference between the Fermi level of the bulk-heterojunction E_(F) and the effective cathode work function Φ_(cathode). The observed changes in V_(BI) are reflected in the magnitude of the change in V_(OC). Further, EQE data reveal the overall device performance enhancement to be due entirely to the inclusion of the surfactant, rather than a beneficial change in photoactive layer morphology.

The following examples are provided for the purpose of illustrating, not limiting, the invention.

EXAMPLES Example 1 The Preparation, Characterization, and Use of Representative Fullerene Surfactants: ETL-1 and ETL-2

In this example, the preparation, characterization, and use of representative fullerene surfactants, ETL-1 and ETL-2, is described. The fabrication and characterization of devices that include the surfactants is also described.

All reactions dealing with air- or moisture-sensitive compounds were carried out using standard Schlenk technique. All ¹H (500 MHz) and ¹³C (125 MHz) spectra were recorded on Bruker AV500 spectrometers. Spectra were reported in parts per million from internal tetramethylsilane (δ 0.00 ppm) or residual protons of the deuterated solvent for ¹H NMR and from solvent carbon (e.g., δ 77.00 ppm for chloroform) for ¹³C NMR. The matrix for MALDI-TOF-MS used 2:1 mixture of alpha-cyano-4-hydroxycinnamic acid (CHCA)/2,5-dihydroxybenzoic acid (DHB) in acetonitrile. Elemental analyses were performed by QTI, Whitehouse, N.J. (www.qtionline.com). AFM images under tapping mode were taken on a Veeco multimode AFM with a Nanoscope III controller. 2,3,4-Tris(2-(2-methoxyethoxy)ethoxy)benzaldehyde and fulleropyrrolidines were synthesized according to literature methods (Benzaldehyde: Nielsen, C B.; Johnsen, M.; Arnbjerg, J.; Pittelkow M.; Mclroy, S P.; Ogilby, P R.; Jrgensen, M. J Org. Chem. 2005, 70:7065. Fulleropyrrolidines and fulleropyrrolidiums: Bosi, S.; Feruglio, L.; Milic, D.; Prato, M. Eur. J. Org. Chem. 2003, 4741). C₆₀ was purchased from American Dye Source. Unless otherwise noted, materials were purchased from Aldrich Inc., and used after appropriate purification.

Synthesis of Fulleropyrrolidiums

A solution of C₆₀ (300 mg, 0.35 mmol), 2,3,4-tris(2-(2-methoxyethoxy)ethoxy)benzaldehyde (478 mg, 1.04 mmol) and sarcosinic acid (111 mg, 1.25 mmol) in chlorobenzene (100 mL) was refluxed under N₂ for 4 h. After evaporation of the solvent, the residue was subjected to chromatograph purification on a silica gel column. Elution with toluene gave little unchanged C₆₀. Fraction containing mono adduct was collected with PhMe/EtOAc (1:2) eluent. One fraction of bisadducts consisting mixture of regioisomers was then collected with EtOAc eluent. Each sample was precipitated from toluene solution with methanol or hexane, and gave monofulleropyrrolidine (115 mg, 27%), bisfulleropyrrolidine (90 mg, 15%).

Quaternization of neutral fulleropyrrolidines was achieved by heating a solution of mono or bis fulleropyrrolidine (0.05 mmol) in chloroform (2 mL) and MeI (1.5 mL) in a screw-topped Schlenk tube under N₂. Reaction mixture was kept at 80 OC for 40 h. After evaporation of the solvent, the product was dissolve in chloroform and precipitated with hexane. After thoroughly washed with n-hexane, black fulleropyrrolidiums, ETL-1 or ETL-2, were obtained in quantitative yield.

Monofulleropyrrolidine.

¹H NMR (500 MHz, CDCl₃): δ 2.78 (s, 3H, NCH ₃), 3.34 (s, 3H, OCH ₃), 3.36 (s, 3H, OCH ₃), 3.40 (s, 3H, OCH ₃), 3.48-3.50 (m, 2H, OCH ₂), 3.53-3.58 (m, 4H, OCH ₂), 3.63-3.80 (m, 10H, OCH ₂), 3.86 (t, J=5.5 Hz, 2H, OCH ₂), 3.05-4.18 (m, 4H, OCH ₂), 4.27-4.32 (m, 2H, OCH ₂), 4.37-4.40 (m, 2H, OCH ₂), 4.94 (d, J=9.5 Hz, 1H, NCH ₂), 5.56 (s, 1H, NCH ₂), 6.77 (d, J=8.5 Hz, 1H, Ar—H), 7.63 (d, J=8.5 Hz, 1H, Ar—H). ¹³C NMR (125 MHz, CDCl₃): δ 39.89, 58.99, 59.00, 59.03, 59.05, 59.18, 59.19, 68.36, 69.25, 69.76, 69.83, 70.21, 70.35, 70.62, 70.72, 70.74, 71.95, 71.98, 72.05, 72.23, 73.18, 75.74, 77.20, 109.34, 123.31, 124.57, 134.82, 136.06, 136.49, 136.59, 139.47, 139.53, 140.12, 140.14, 141.19, 141.58, 141.67, 141.89, 141.99, 142.08, 142.11, 142.16, 142.28, 142.29, 142.54, 142.57, 142.63, 142.65, 143.00, 143.08, 144.36, 144.45, 144.61, 145.11, 145.12, 145.18, 145.23, 145.26, 145.31, 145.55, 145.76, 145.94, 146.07, 146.09, 146.12, 146.20, 146.27, 146.77, 146.95, 147.30, 152.20, 152.47, 154.15, 154.33, 155.16, 156.87. MALDI-TOF-MS (+): calcd. for [C₈₄H₄₁NO₉]⁻, 1208.225. found. [M]⁻, 1207.893.

Bisfulleropyrrolidine.

¹H NMR (500 MHz, CDCl₃): δ 2.55-2.88 (m, NCH ₃), 3.29-3.40 (m, OCH ₃), 3.42-4.00 (m, OCH ₂&OCH ₃), 4.06-4.68, 4.92-5.57, 5.74-5.75, 6.52-6.98, 7.35-7.49, 7.59-7.69, 7.73-7.88, 8.00-8.03; ¹³C NMR (125 MHz, CDCl₃): δ 39.66-39.83 (m, NCH₃), 53.21-53.43 (m), 58.97-59.22 (m), 68.17-68.43, 69.42, 69.70-69.94, 70.14-70.91, 71.90-72.35, 72.92-73.50, 75.34-76.00, 77.40-77.66, 109.12-109.48, 123.58-123.98, 124.42-124.61, 134.87, 136.53, 139.39, 140.76-141.93, 142.14, 142.00, 142.23, 142.30, 142.37, 142.51, 142.58, 142.95, 142.97, 143.38, 143.39, 143.58, 144.12, 144.36, 144.85, 144.96, 145.08, 145.21, 145.26, 145.44-145.74, 146.05, 146.07, 147.25, 147.47, 147.72, 147.84, 148.64, 148.77, 149.03, 150.75-151.39, 151.97-152.83, 153.66, 154.28-154.98, 155.54; MALDI-TOF-MS (+): calcd. for [C₁₀₈H₈₂N₂O₁₈], 1695.809. found. [M-I]⁺, 1695.929.

Fulleropyrrolidium ETL-1.

¹H NMR (500 MHz, CDCl₃): δ 3.36 (s, 3H, OCH ₃), 3.38 (s, 3H, OCH ₃), 3.52-3.56 (m, 7H, OCH ₂&OCH ₃), 3.67-3.77 (m, 8H, OCH ₂), 3.80-3.84 (m, 2H, OCH ₂), 3.89 (m, 2H, OCH ₂), 3.97 (s, 3H, NCH ₃), 4.02 (d, J=8.5 Hz, 2H, OCH ₂), 4.20-4.39 (m, 4H, OCH ₂), 4.48 (s, 3H, NCH ₃), 4.66-4.68 (m, 2H, OCH ₂), 5.80 (d, J=12.5 Hz, 1H, NCH ₂), 6.84 (d, J=13.0 Hz, 1H, NCH ₂), 6.88 (d, J=9.0 Hz, 1H, Ar—H), 7.28 (d, J=13.0 Hz, 1H, NCH ₂), 7.71 (d, J=8.5 Hz, 1H, Ar—H); ¹³C NMR (125 MHz, CDCl₃): δ 45.69, 53.44, 59.04, 59.07, 59.08, 59.28, 67.89, 68.44, 69.38, 69.96, 70.43, 70.54, 70.69, 70.72, 71.42, 71.65, 71.93, 71.97, 72.53, 72.56, 73.13, 73.64, 78.60, 108.66, 111.57, 127.48, 134.10, 134.75, 135.52, 136.11, 139.03, 139.87, 139.98, 140.26, 140.93, 141.24, 141.38, 141.43, 141.45, 141.62, 141.82, 142.03, 142.09, 142.11, 142.13, 142.35, 142.39, 142.51, 142.52, 142.76, 142.84, 142.96, 143.01, 143.12, 143.33, 144.19, 144.23, 144.36, 144.42, 144.82, 144.89, 145.14, 145.26, 145.30, 145.45, 145.54, 145.61, 145.66, 145.77, 145.82, 145.96, 146.02, 146.13, 146.18, 146.35, 146.40, 147.42, 147.56, 149.32, 150.51, 151.18, 152.66, 153.66, 153.83, 155.77; MALDI-TOF-MS (+): calcd. for [C₈₅H₄₄NO₉]⁺.I⁻, 1350.16. found. [M-I]⁺, 1222.144; Anal. Calcd for C₈₅H₄₄NO₉: C, 75.61; H, 3.28; N, 1.04. Found: C, 73.29; H, 2.76; N, 0.76.

Fulleropyrrolidium ETL-2 (Mixture of Regioisomers).

¹H NMR (500 MHz, CDCl₃/CD₃OD): δ 3.32-3.40 (m, OCH ₃), 3.42-4.03 (m, OCH ₂&OCH ₃), 4.12-4.50 (m, OCH ₂), 4.56-4.61 (m, OCH ₂), 4.70-4.72 (m, OCH ₂), 5.36-5.69 (m, NCH ₂), 6.02-6.07, 6.68-6.97, 7.04-7.13, 7.20-7.21, 7.32-7.34, 7.37-7.61, 7.77-7.89, 7.98-8.04, 8.10-8.14, 8.27-8.31; ¹³C NMR (125 MHz, CDCl₃): δ 45.28-46.47 (m), 53.21-53.43 (m), 58.99-59.49 (m), 66.06, 66.83, 68.45, 68.49-69.41, 69.51-70.89, 71.27-71.52, 71.96-72.06, 72.46-72.67, 73.55-73.84, 78.65, 78.75, 109.24, 109.35, 109.48, 111.29, 111.44, 136.24, 136.67, 140.04, 140.44, 140.84, 140.96, 141.13, 141.35, 141.58, 141.60, 141.67, 141.73, 141.77, 141.81, 141.83, 141.94, 142.14, 142.17, 142.21, 142.32, 142.38, 142.40, 142.51, 142.61, 145.38, 145.48, 145.59, 146.14, 146.20, 147.21, 147.40, 147.53, 147.79, 147.96-148.09, 148.40, 148.70-148.82, 149.08-149.32, 150.06, 151.57, 153.67-153.76, 155.68-155.89; MALDI-TOF-MS: calcd. for [C₁₁₀H₈₈N₂O₁₈]²⁺.2I⁻, 1979.69. found [M-2I-NMe₃]⁺ 1666.278; Anal. Calcd for C₁₁₀H₈₈I₂N₂O₁₈: C, 66.74; H, 4.48; N, 1.42. Found: C, 66.07; H, 4.23; N, 1.35.

CV Measurements

Cyclic voltammetry (CV) measurements were carried out in a one-compartment cell under N₂, equipped with a glassy-carbon working electrode, a platinum wire counter electrode, and an Ag/Ag⁺ reference electrode. Measurements were performed in THF solution containing tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte with a scan rate of 100 mV/s. All potentials were corrected against Fc/Fc⁺. Due to close vicinity of the electron-deficient cationic nitrogen to the fullerene core, the LUMO level of ETL-1 to that of ETL-2 has a small difference in 0.03 eV.

Fabrication and Characterization of PSCs

[6,6]-Phenyl-C61 (or C71)-butyric methyl ester was purchased from American Dye Source. PEDOT:PSS (Baytron P VP AI 4083) was purchase from H. C. Stark. Materials were used as received. The fullerene surfactant solutions in methanol were sonicated for 2 hrs prior to spin-coating in ambient at 5000 RPM. The surfactant layer thickness was about 8-10 nm as measured by AFM. The ITO substrates were cleaned by ultrasonication in acetone for 15 min, followed by manual scrubbing with detergent and deionized water, then sonication in deionized water and isopropanol for 15 min each. The substrates were blown dry under a nitrogen stream and immediately exposed to air plasma for 20 seconds. A 40 nm thick layer of PEDOT:PSS was spin coated onto each substrate and subsequently annealed in air at 140° C. for 30 min. The mixture of PIDT-PhanQ:PC₇₁BM in o-dichlorobenzene (20 mg/ml, 1:3, w:w) was then spin-coated on the PEDOT:PSS layers at 800 RPM, and subsequently annealed at 110° C. for 10 min under nitrogen atmosphere to obtain a film thickness approximately 80 nm. After fullerene surfactant solutions was spin coated on the BHJ layer. The substrates were then transferred back into the glovebox and annealed at 110° C. for 5 min. Finally, aluminum (100 nm) or calcium (30 nm) topped with aluminum (100 nm), or silver (100 nm) was thermally evaporated onto the active layer through shadow masks.

Photocurrent-voltage (J-V) measurements were performed using a Keithley 4200 in a nitrogen-filled glove box under AM1.5 illumination conditions at intensity of 100 mW/cm². A NREL certified silicon photodiode with a KG5 filter was used to calibrate. Device EQE spectra were obtained in air by comparison to a known AM1.5 reference spectrum for a calibrated silicon photodiode.

Organic Field-Effect Transistors

Top contact OFETs were fabricated as typical top contact, bottom gate devices on silicon substrates. Heavily doped p-type silicon <100> substrates from Montco Silicon Technologies INC. with a 300 nm (±5 nm) thermal oxide layer acted as a common gate with a dielectric layer. After cleaning the substrate by sequential ultrasonication in acetone, methanol, and isopropyl alcohol for 15 min flowed by air plasma treatment, the different fullerene surfactant films were spin-coated from a 0.5 wt % chloroform solution in ambient. Interdigitated source and drain electrodes (W=1000 μm, L=12 μm) were defined by evaporating a 50 nm Au film through a shadow mask from the resistively heated Mo boat at 10⁻⁶ Torr. OFET characterization was carried out in a N₂-filled glovebox using an Agilent 4155B semiconductor parameter S6 analyzer. The field-effect mobility was calculated in the saturation regime from the linear fit of (I_(ds))_(1/2) VS V_(gs). The threshold voltage (V_(t)) was estimated as the x intercept of the linear section of the plot of (I_(ds))_(1/2) VS V_(gs). The sub threshold swing was calculated by taking the inverse of the slope of I_(ds) VS V_(gs) in the region of exponential current increase.

Work Function Measurements by XPS

Samples for work function analysis were prepared on glass substrates coated with ITO to ensure good electrical contact. Work functions were measured with a PHI Versa Probe X-ray photoelectron spectrometer (ULVAC-PHI, Kanagawa, Japan) employing a monochromatic focused Al—K_(α) X-ray source and hemispherical analyzer. The Au 4f_(7/2) (84.00 eV) and Cu 2p_(3/2) (932.66 eV) photoemission peaks were used to calibrate the binding energy scale. A bias voltage (−5 V) was applied to the sample, and the location of the secondary electron cut-off was determined at normal emission by a linear extrapolation to the background level. To account for the instrument width, 0.14 eV were added to the work function values thus obtained. This procedure gives a work function for argon ion sputtered gold foil of 5.17 eV.

TABLE 5 Comparison of WF of cathodes. Al ETL-1/Al ETL-2/Al Secondary electron emission (eV) 1477.54 1478.16 1478.32 Work-Function (eV) 4.20 3.66 3.42

Example 2 The Preparation and Characterization of Representative Photovoltaic Devices with Fullerene Surfactant-Containing Interfacial Layer

In this example, the preparation and characterization of representative photovoltaic devices with a fullerene surfactant-containing layer intermediate the active layer and cathode is described.

Fabrication of Photovoltaic Devices

ITO-coated glass substrates (15 Ωsq⁻¹) were cleaned sequentially by sonication in detergent and deionized water, acetone and isopropanol. After drying under a N₂ stream, substrates were air-plasma treated for 30 s. A about 35 nm layer of PEDOT:PSS (Baytron® P VP Al 4083, filtered through a 0.45 μm nylon filter) was spin-coated onto the clean substrates at 5 kRPM and annealed at 140° C. for 10 min. The substrates were transferred to a N₂-filled glovebox where a homogeneously blended solution of PIDTPhanQ:PC₇₁BM (40 mg/ml in o-dichlorobenzene stirred overnight in glovebox, 1:3 polymer:fullerene by weight) was spin-coated at 2 k RPM, producing an active layer about 100 nm thick, and annealed at 110° C. for 10 min. Substrates requiring a layer of fullerene surfactant were briefly transferred out of the glovebox (total ambient exposure<10 min) and about 2-5 nm thick film of C₆₀-bis surfactant (1 mg/ml in methanol) was spin-coated at 5 k RPM. The substrates were then transferred back into the glovebox and annealed at 110° C. for 5 min to drive off any remaining solvent prior to metal deposition. Metal electrodes were deposited at a base pressure<1×10⁻⁶ Torr through a shadow mask, defining an active device area of 4.64 mm². Ag and Cu were deposited at a rate of 1 Å s¹ and Al was deposited at a rate of 4 Å s⁻¹.

Preparation of XPS Samples

ITO-coated glass substrates were prepared as above without air-plasma treatment. Al, Ag, and Cu were deposited over the entire substrate surface at a rate of 1 Å s⁻¹. Substrates requiring a thin layer of fullerene surfactant were transferred out of the glovebox and a solution of C₆₀-bis surfactant was spin-coated from methanol using the same conditions as above. After transfer back into the glovebox, all substrates were heated at 70° C. for 5 min to evaporate any remaining methanol prior to being sealed with parafilm in 20 ml glass vials under N₂ for transport to the XPS.

Measurement and Characterization

J-V characteristics of the unencapsulated devices were measured in ambient conditions using a Keithley 2400 source meter under AM 1.5 G (100 mW cm⁻²) irradiation simulated by an Oriel xenon lamp (450 W). AM 1.5 G illumination was confirmed by means of calibration to a standard silicon photodiode (Hammamatsu) which can be traced to the National Renewable Energy Laboratory. External quantum efficiency spectra were obtained by measuring the photocurrent response of the device using chopped, monochromated light from the same xenon lamp in conjunction with a Stanford Research Systems SR830 lock-in amplifier under ambient conditions. Mott-Schottky analysis was performed in a N₂-filled glovebox in the dark using a Signatone probe station interfaced with a Hewlett-Packard HP4284A LCR meter. The 1 kHz AC field applied during measurement was kept at an amplitude of 25 mV to maintain response linearity. Capacitance-voltage characteristics measured thusly were obtained using devices prepared as above with an active area of 10.08 mm². Work function determination via XPS is described below. Briefly, the secondary electron cutoff (SEC) spectrum of each sample was measured under ultra-high vacuum (<5×10⁻⁹ Torr) using a PHI 5000 VersaProbe (Ulvac-Phi, Inc.) employing a focused, monochromated Al K-α x-ray source and a hemispherical analyzer. Proper referencing of the SEC edge to that of Ar⁺ ion sputter-cleaned, polycrystalline gold allowed for accurate determination of the sample work functions with a reproducibility of about 0.05 eV.

Cyclic Voltammetry Measurements

Cyclic voltammetry measurements were carried out under N₂ in a one-compartment cell equipped with a glassy carbon working electrode, a platinum wire counter electrode, and an Ag/Ag⁺ reference electrode. Measurements were performed in THF solution containing tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte with a scan rate of 100 mV/s. All potentials were corrected against the Fc/Fc⁺ couple and LUMO levels were estimated using the following equation: LUMO=−(4.8+E_(1/2) ^(red1)) eV.

Work Function Determination

Work function values were obtained following a modified method previously described (M. M. Beerbom et al., Journal of Electron Spectroscopy and Related Phenomena 152, 2006, 12-17). The spectrometer's analyzer was calibrated according to the manufacturer's guidelines to yield photoemission lines of Ar⁺ ion sputter-cleaned Cu and Au foils for Cu 2p 3/2 and Au 4f 7/2 at 932.62 eV and 83.96 eV, respectively, following ISO 15472 (M. P. Seah, Surf. Interface Anal., 31, 2001, 721-723). This procedure ensures the linearity of the binding energy scale for the instrument, extrapolated out to the secondary electron cutoff (SEC) near the photon energy of the system (1486.6 eV for monochromated Al K-α x-rays). SEC spectra were measured at an x-ray power of 25 W and 15 kV acceleration at normal emission. For all SEC spectra a bias of −15V was applied during measurement to ensure sufficient separation of the sample SEC and that of the analyzer. Under these conditions a SEC value of 1466.24 eV for clean, polycrystalline gold was obtained, corresponding to a work function of 5.36 eV. Because the Cu and Au core level spectra mentioned above are referenced to the Fermi level, set at zero binding energy, the work function of Au was obtained by Φ_(Au)=(hυ−qV_(app)−E_(SEC)) where hυ is the x-ray photon energy, V_(app) is the applied bias and E_(SEC) is the position of the secondary electron cutoff on the binding energy scale. Ideally, the SEC edge should be a step function at 0 K, however experimental conditions include thermal and instrumental broadening. Hence, the position of the SEC is taken as the local maximum of the first derivative of the SEC feature. FIG. 11 shows the SEC spectrum of clean, polycrystalline Au foil and its corresponding first derivative. Once the work function of clean Au has been obtained thusly, all other sample work functions can be derived simply from their SEC positions obtained via the first derivative method as Φ_(sample)=(E_(SEC, Au)−E_(SEC, sample))+Φ_(Au). FIGS. 12A-12C show the SEC spectra for Al, Ag, and Cu with and without C₆₀-bis. FIG. 14C includes the SEC spectrum of clean Au foil as a reference.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A photovoltaic device comprising: (a) a first electrode; (b) an active layer disposed on a surface of the first electrode; (c) a layer comprising a compound disposed on a surface of the active layer opposite the first electrode, wherein the compound comprises (i) a fullerene group; (ii) one or more cationic nitrogen centers covalently coupled to the fullerene group; (iii) one or more hydrophilic groups covalently coupled to the fullerene group; (iv) one or more counter ions associated with the cationic nitrogen center; and (d) a second electrode disposed on a surface of the layer comprising the compound opposite the active layer.
 2. The photovoltaic device of claim 1, wherein the fullerene group is selected from the group consisting of C₆₀, C₇₀, C₇₆, C₇₈, C₈₂, C₈₄, and C₉₂ fullerene groups.
 3. The photovoltaic device of claim 1, wherein the cationic nitrogen center is a quaternary amine group.
 4. The photovoltaic device of claim 1, wherein the hydrophilic group is a polyether group.
 5. The photovoltaic device of claim 4, wherein the polyether group is a polyalkene oxide group.
 6. The photovoltaic device of claim 5, wherein the polyalkene oxide group is a polyethylene oxide group having the formula —(CH₂CH₂O)_(n)—, where n is from 1 to about
 20. 7. The photovoltaic device of claim 1, wherein the compound further comprises an anionic center.
 8. The photovoltaic device of claim 7, wherein the anionic center is selected from the group consisting of sulfonate (SO₃ ²⁻) and carboxylate (—CO₂ ⁻) groups.
 9. The photovoltaic device of claim 1, wherein the fullerene group is selected from the group consisting of a mono-fulleropyrrolidium group and a bis-fulleropyrrolidium group.
 10. The photovoltaic device of claim 1, wherein the compound has the structure:

wherein F is a fullerene group; B is a N-containing ring having from 5-7 ring atoms; R₁ and R₂ are independently selected from the group consisting of a polyalkylene oxide and a C1-C20 alkyl optionally substituted with an anionic center; Ar is —C₆H₅-PEO, wherein —C₆H₅-PEO is selected from the group consisting of mono-, di-, tri-, and tetra-PEO substituted phenyl; and A⁻ is a counter ion associated with the cationic nitrogen center.
 11. The photovoltaic device of claim 1, wherein the compound has the structure:

wherein F is a fullerene group; B is a N-containing ring having from 5-7 ring atoms; R₁ and R₂ are independently selected from the group consisting of a polyalkylene oxide and a C1-C20 alkyl optionally substituted with an anionic center; Ar is —C₆H₅-PEO, wherein —C₆H₅-PEO is selected from the group consisting of mono-, di-, tri-, and tetra-PEO substituted phenyl; and A⁻ is a counter ion associated with the cationic nitrogen center.
 12. The photovoltaic device of claim 1, wherein the compound has the structure:


13. The photovoltaic device of claim 1, wherein the compound has the structure:

wherein R is independently selected from the group consisting of C1-C20 straight chain and branched alkyl; PEO is an alkylene oxide group independently selected from the group consisting of polyethylene oxide having the formula —(CH₂CH₂O)_(n)—, where n is from 1 to about 20 or polypropylene oxide having the formula —(CH(CH₃)CH₂O)_(n)—, where n is from 1 to about 20; C₆H₅-PEO is selected from the group consisting of mono-, di-, tri-, and tetra-PEO substituted phenyl; and A⁻ is a counter ion selected from the group consisting of fluoride, chloride, bromide, iodide, trifluoromethyl sulfonyl (CF₃SO₃ ⁻), tetrakis(imidazolyl)borate (BIm₄ ⁻), and tetrakis(3,5-bis(trifluoromethyl)phenyl]borate (TFPB⁻).
 14. The photovoltaic device of claim 1, wherein the compound has the structure:

wherein R is independently selected from the group consisting of C1-C20 straight chain and branched alkyl; PEO is an alkylene oxide group independently selected from the group consisting of polyethylene oxide having the formula —(CH₂CH₂O)_(n)—, where n is from 1 to about 20 or polypropylene oxide having the formula —(CH(CH₃)CH₂O)_(n)—, where n is from 1 to about 20; C₆H₅-PEO is selected from the group consisting of mono-, di-, tri-, and tetra-PEO substituted phenyl; and A⁻ is a counter ion selected from the group consisting of fluoride, chloride, bromide, iodide, trifluoromethyl sulfonyl (CF₃SO₃ ⁻), tetrakis(imidazolyl)borate (BIm₄ ⁻), and tetrakis(3,5-bis(trifluoromethyl)phenyl]borate (TFPB⁻).
 15. The photovoltaic device of claim 1 further comprising a charge transport layer intermediate the first electrode and the active layer.
 16. The photovoltaic device of claim 1, wherein the active layer comprises an active fullerene material. 